1. Field of the Invention
The present invention relates to integrated circuits and to integrated circuit manufacturing methods. More specifically, the present invention relates to the structure of the metal tracks of integrated circuits.
2. Discussion of the Related Art
Integrated circuit design rules impose minimum distances for the elements of an integrated circuit and minimum intervals between the integrated circuit elements, especially the metal tracks which extend between insulating layers and/or at the surface of an insulating layer and conductive vias which generally extend through insulating layers. A metal track may have any shape and, in particular, may correspond to a metal pad with a square or rectangular base. The minimum track dimension may be defined as the smallest track dimension that can be used, apart from the thickness. The minimum track dimension especially depends on the techniques implemented in the integrated circuit manufacturing.
The current tendency is to decrease the dimensions of integrated circuits. For certain integrated circuits, the dimensions of the metal tracks defined on design of the integrated circuit define the final dimensions of the integrated circuit. Such may be the case for metal tracks of the first metallization level of the non-volatile ROMs (read-only memories) which are programmed on manufacturing or at the first use of the integrated circuit and which can be neither erased, nor reprogrammed afterwards. Indeed, in this case, the tracks of the first metallization level at least partly correspond to pads and the step of the pads define the size of the memory.
Among conventional methods for manufacturing metal tracks of an integrated circuit, methods with aluminum-copper tracks for which all the integrated circuit tracks are based on an alloy containing aluminum and a small amount of copper, and methods with copper tracks, for which all the integrated circuit tracks are made of copper can be distinguished. In the following description, expression “aluminum track” indifferently means a track made of aluminum only or based on an alloy of aluminum and of a small amount of one or of several other materials, for example, copper.
The minimum dimension to be provided for an aluminum track is greater than the minimum dimension to be provided for a copper track due to the techniques implemented in the forming of aluminum tracks. Thereby, when the final dimensions of an integrated circuit are imposed by the dimensions of the metal tracks of the first metallization levels, the implementation of a copper track method instead of an aluminum-copper track method enables to decrease the integrated circuit dimensions. However, the implementation cost of a copper track method is greater than the implementation cost of an aluminum-copper track method, which limits the use thereof.